In a wireless communication network a base station communicates with a user equipment (UE) which is actively located within an area of radio coverage of the base station and comprises a receiver for receiving signals from the base station. Some examples of user equipments include cell phones, PDA's, laptops, mobile broadband modems etc.
User equipments are commonly at different locations within the area of radio coverage with correspondingly different received signal strength and interference levels. Consequently, some user equipments can receive data at much higher data rates than other user equipments. In order to optimally utilize the data transmission time, it is desirable to ensure that the base station transmits to each user equipment in a manner tailored to the channel conditions experienced by the user equipment. Tailoring such a transmission is called link adaptation which is a term used to denote the matching of the modulation, coding and other signal and protocol parameters to the conditions on the radio link.
Moreover, in a spectrum aggregated or multi-band system, several frequency bands, continuous or non-continuous, are allocated for the communication with one UE. The modulation and access format within the band could be of any kind, e.g., orthogonal frequency division multiplexing (OFDM), code division multiplexing (CDMA), frequency division multiplexing (FDMA), etc. In this context, one band is referred to as one “component band”.
To enable the base station to perform link adaptation and user equipment scheduling, the user equipment has to feedback a channel quality measure, which may be a so called channel quality indicator (CQI), based on its estimated channel condition. Hence, in various kinds of wireless communication systems, it is currently a common measure to estimate and report a CQI which can be a value, or several values, representing a measure of channel quality for a given radio channel. Usually, a low CQI value is indicative of a channel with low quality and vice versa. A CQI for a channel can be computed by making use of performance metric, such as a signal-to-noise ratio (SNR), signal-to-interference plus noise ratio (SINR), signal-to-noise plus distortion ratio (SNDR) etc. of the communication channel. These ratio-values and other values can be measured for a given channel and then be used to compute a CQI for the channel. Other factors that may be taken into account in CQI are performance impairments, such as Doppler shift, channel estimation error, interference etc.
It should be noted that CQI often does not explicitly indicate the channel quality, but rather the data rate supported by the user equipment given the current channel conditions. More specifically, in this case the CQI may be seen as a recommended data rate over the channel.
The channel quality measure may be represented by the CQI value or by any other measure indicating the quality of a wireless communication channel.
If the base station and UE has a single transmit antenna, the use of channel quality measure, or CQI estimation, for link adaptation and/or user equipment scheduling is rather straight forward. However, today more complex communications systems are employed, such as those making use of multiple-input multiple output (MIMO) and space-time coded systems, for which the channel quality measure may also be dependent on a receiver type in the user equipment. This means that for multi band, greater flexibility in channel quality measure, link adaptation and user equipment scheduling is offered, which implies that improvements would prove beneficial in the art. Further background art is reflected by published patent applications WO2003/039032, US2006/165188, US2008/139153, WO2008/030661, and WO2008/051038.